I think they are all overpriced on ebay and will never make ROI... but as a hobbyist, thats probably the only way to buy a 1TH machine quickly like a KnC. For ROI, I'll stick with products that actually deliver on time like Antminer S1s instead of Ebay. Also beware of scams on ebay especially when people try to sell you their pre-orders from BFL and such. These forums are full of sorry tales of getting scammed on them.
And use common sense. If a machine was making ROI and tons of BTC, why on earth would anyone sell it to you?
As far as USB ones go, remember you can buy them brand new for $15 a piece at today's BTC/USD price. Anything above that is you throwing your money away. It's still too pricey for me (price per ghs) but someone is buying these for sure
https://bitmaintech.com/productDetail.htm?pid=00020140227111546291QgbvQbVr06FB